This invention deals generally with wear and corrosion resistant coatings on metal and more specifically with a coating for plastic injection molds.
Over the last decade, diamond-like carbon coatings deposited on various tools and parts have gained wide acceptance for wear and corrosion protection. The diamond-like carbon coatings demonstrate a unique combination of properties that include high hardness in excess of 2,000 GPa and extremely low coefficients of friction and surface tension. Furthermore, they are chemically inert when in a variety of aggressive environments and have an aesthetically attractive appearance. Such diamond-like carbon coatings are amorphous, which means that they do not have long range repeatability of atomic positions in their crystalline structure.
In recent years diamond-like carbon films have been introduced for coating molds that are used for plastic injection molding where the anti-sticking properties of the diamond-like carbon coatings allow for reduced downtime, longer mold life, and improved product quality. Diamond-like carbon films have been particularly beneficial when used for the protection of the working surfaces of the molds and the mold parts used for manufacturing optical media discs such as CDs, DVDs, and the like. Although discs manufactured with diamond-like carbon film coated molds generally outperform those from uncoated or titanium nitride coated molds, the field performance of diamond-like carbon coated molds varies for different methods of depositing the diamond-like carbon.
Diamond-like carbon coating are generally divided into two broad categories, those containing carbon with less than 1% of other constituents and those containing carbon with greater than 10% of hydrogen. As a rule, diamond-like carbon coatings deposited by sputtering or arcing a carbon target produce hydrogen-free coatings, while films obtained by the plasma assisted breakdown of hydrocarbon gases contain hydrogen. The most common method of depositing diamond-like carbon coatings with hydrogen is by generating a radio frequency with a plasma inside a vacuum chamber where a substrate acts as one electrode and the vacuum chamber body operates as the other electrode. Diamond-like carbon coatings with hydrogen generally have a lower hardness.
One of the drawbacks of diamond-like carbon coatings is the high magnitude of inherent residual stress, which is largely responsible for the problems with film delamination, and the residual stress is generally higher in hydrogen-free diamond-like carbon films. The prior art discloses various underlayers to be used to form a transitional layer that assists in absorbing some of the stress, thus increasing the integrity of the entire coating. The most common underlayer is a layer of titanium or silicon with a thickness of 0.2 to 1.5 microns.
However, even with the underlayer, residual stress leads to a specific failure mode of the diamond-like carbon films used on molds for manufacturing optical discs. In such applications, as the film experiences normal operational wear and the thickness is reduced to 0.3 to 0.5 microns, the coating develops cracks which ultimately break through the entire coating. Since the overall initial thickness of diamond-like carbon coatings rarely exceeds 1.5 microns and typically is in the range of between 1.0 and 1.5 microns, molds can lose up to 50% of their useful life because of premature fracture of the film.
This type of failure can occur in both the signal and the stamper sides of optical media disc molds. Such discs are manufactured by injecting molten resin material into the cavity formed by a signal side mold, a cavity ring, and a stamper supported by a stamper side mold. Failure in the wear resistant film can mean that a mold produces 20% to 50% fewer discs.
It would be very beneficial to have a coating for optical media molds that has a lower wear rate than those presently available and is not subject to premature failure when a significant thickness of the film still remains.
The present invention uses a film of carbon nitride on mold parts to dramatically increase the useful life of the mold. The film is an amorphous carbon nitride film with a nitrogen content in the range of between 2% and 45% and can be used on molds or mold parts used in optical disc manufacture as a single layer coating or as multiple layers in combination with one or more layers of diamond-like carbon.
In regard to carbon nitride, the compound xcex2-C3N4 has been predicted theoretically, but has not actually been produced to date. The amorphous material used in the present invention and produced by the apparatus and methods disclosed is carbon nitride with a mixture of lattices in varying ratios of carbon and nitrogen. Such material is often designated by the symbol CNx.
Operational testing has confirmed that molds with a single layer of carbon nitride or alternate layers of carbon nitride and diamond-like carbon deposited according to the methods of the invention are not as prone to premature failure as molds with diamond-like carbon films alone. The typical wear pattern of the film of the invention is similar to metal-based coatings like TiN, however, the wear rate is dramatically lower. Consequently, the invention yields increased useful life for the molds upon which it is used while maintaining high quality of the products produced by the molds.
The coating of the invention has been produced by both pulsed arc and radio frequency chemical vapor deposition methods which yield carbon nitride coatings with 2% to 45% nitrogen and 0.3 to 5 microns thickness. The apparatus and methods used for producing the coating of the invention are described herein. Pulsed arc deposition of carbon nitride involves the generation of a carbon plasma by the use of a pulsed discharge. In order to generate the carbon plasma, an arc discharge is initiated on the surface of a carbon target. This is similar to the method used with metal targets. However, with metal targets movement of the arc occurs inherently in a random manner With the arc discharge spontaneously extinguishing itself and then re-igniting in a new location, while in the case of a carbon target the spot does not normally extinguish itself or move. Because the amount of material an arc evaporates from any location depends on the time the arc remains in that location, a stationary arc would lead to rapidly burning through the target material followed by catastrophic arcing of the material of the target holder.
In order to prevent destruction of the target material, the pulsed arc method is used to move the arc spot around on the carbon target. The method involves continuously extinguishing and re-igniting the arc by cycling the arc and ignition power supplies on and off, creating a series of pulses at a pulse rate of between 10 and 100 Hz. Under such circumstances each pulse ignites the arc at a different location on the target, thus preventing the catastrophic erosion of the carbon target.
To produce the carbon nitride coating of the invention, a carbon plasma is produced in a vacuum chamber, nitrogen gas is introduced into the vacuum chamber, and a negative voltage in the range of between 100 and 600 volts is applied to the surface to be coated. These conditions form a uniform layer of carbon nitride film with a thickness of 0.5 to 5 microns, and the film has a nitrogen content of 2 to 45%.
An alternate apparatus and method for producing the carbon nitride coating of the invention by radio frequency chemical vapor deposition is similar to the apparatus and method disclosed in U.S. patent application Ser. No. 09/877,451 filed on Jun. 11, 2001 by J. Hans Richter et al and entitled xe2x80x9cDiamond-Like Carbon Coating for Optical Media Moldsxe2x80x9d. The disclosure of that concurrent patent application is incorporated herein by reference and made a part of this application.
The prior application describes an apparatus and method of producing a defect free underlayer coating onto an optical media mold, and depositing a diamond-like carbon coating upon the underlayer. The apparatus uses a hollow cathode electron beam generator and a rotating crucible containing the material for the underlayer. The present invention uses the same method and apparatus within a vacuum chamber to produce the underlayer on the mold parts, but after the underlayer coating is completed, a mixture of acetylene and nitrogen is introduced into the vacuum chamber while a radio frequency is used to initiate a plasma. The plasma and particular gas mixture form carbon nitride by breaking down the acetylene and combining the carbon from the acetylene with the nitrogen. Nitrogen content of the coating deposited using the radio frequency method may be varied by either changing the ratio of partial pressure of the reactive gases or by varying the radio frequency power input.
Even when only a stamper side mold producing conventional audio compact discs was coated with the carbon nitride film of the invention by the pulsed arc method of the invention, the coating of the invention improved the yield of the mold before failure by 50% to 500%, an increased yield of several million discs, over the typical previous coating of diamond-like carbon.
The invention thereby provides a dramatic increased lifetime of the molds and thus reduces the cost of manufacturing.